Air conditioning and refrigeration systems typically operate in a cycle which employs a series of functions, including a compression step wherein low-pressure refrigerant gas is compressed to a high-pressure state and a condensation step in which the gas is converted to a liquid, accompanied by a release of heat.. The condensed liquid is then forced under pressure through an expansion device, partially converting the liquid to a gaseous state, after which the refrigerant is passed through a second heat exchanger or evaporator stage where heat from the surrounding air is transferred to the refrigerant, resulting in cooling of the air. Low pressure gas produced by evaporation is then recycled to the compressor.
Air conditioning systems generally use one of three different types of expansion devices; a thermostatic expansion valve, a capillary tube or tubes with a precise diameter or length or a fixed orifice piston check valve having a cylindrical chamber in which a piston with an axial bore is mounted for movement responsive to the direction of fluid flow. This invention is concerned with the third type, that is, expanders using a piston orifice check valve. Orifice pistons in these devices move from a position at the rearward end of the chamber to a position at the opposite or forward end, where the periphery of the piston is sealed by fluid-tight engagement of mating surface of the piston and chamber so that forward flow of the refrigerant is restricted to the central orifice of the piston. The orifice is sized according to system specifications to provide a selected pressure drop within the refrigerant flow stream. Flow in the reverse direction toward the rearward end of the chamber would cause the piston to move out of sealing engagement with the chamber, allowing unrestricted movement of the fluid through flutes or grooves around the periphery of the piston. Reverse flow would thus be provided as would be required for systems using a heat pump cycle. Piston orifice expansion devices of this type are exemplified by U.S. Pat. No. 5,894,741, issued Apr. 20, 1999, to Durham, et al.
A problem associated with piston orifice check-valve expansion devices is that the pressure which builds up across the piston when the system is operating is allowed to bleed down through the orifice when the system cuts off, which occurs periodically. This results in a requirement for use of additional energy to bring the system back to operating pressure upon start up. Providing a means for maintaining operating pressure differential across the piston during the off cycle would result in higher system efficiency and a consequent saving of energy costs.
Solenoid valves have been incorporated into the same valve body for certain types of expansion devices, as is disclosed in U.S. Pat. No. 5,588,590, issued Dec. 31, 1996, to Sakakibara, et al. The expansion valve of this device is a capillary type device, and the patent is concerned with eliminating impact noise resulting from "water hammer" by placing a solenoid valve between primary and secondary ports. The solenoid valve is not used to maintain a pressure different across the expander upon shutdown, the combined valve instead having an equalizer hole to reduce pressure differential. This patent makes reference to a prior Japanese patent publication disclosing use of a solenoid valve provided to the evaporator to shut off refrigerant flow to evaporators of a multi-stage system which are not in use, thus resulting in a saving of energy. The cited patent, however, does not disclose a combined device having a piston orifice valve and a solenoid valve disposed in a single integrated housing and functioning as a unit. Further improvements directed to prevention of migration of refrigerant between areas of different pressures would be desirable.